AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 277 



vegetation or by leachino- out of soluble salts retained in the deeper 

 lying- parts. The matter is stated as follows by Mendeleeff:^ 



''The primary rocks contain an almost equal proportion of potassium 

 and sodium. But in sea-water the compounds of the latter metal pre- 

 dominate. It may be asked. What became of the compounds of potas- 

 sium in the disintegration of the primary rocks if so small a quantity 

 went to the sea- waters 



"They remained with the other products of the decomposition of 

 the primary rocks. When granite or any other similar rock-forma- 

 tion is disintegrated there are formed, besides the soluble substances, 

 also insoluble substances — sand and finely divided clay, containing 

 water, alumina, and silica. This clay is carried away by the water, 

 and is then deposited in strata. It, and especially its admixture with 

 vegetable remains, retains compounds of potassium in a greater quan- 

 tity than those of sodium. This has been proved Avith absolute cer- 

 tainty to be the case, and is due to the absorptive power of the soil. 

 If a dilute solution of a potassium compound be filtered through com- 

 mon mold used for growing plants, containing clay and the remains 

 of vegetable decomposition, this mold will be found to have retained 

 a somewhat considerable percentage of the potassium compounds. If 

 a salt of potassium be taken, then, during the filtration, an equivalent 

 quantity of a salt of calcium — which is also found, as a rule, in soils — is 

 set free. Such a process of filtration through finely divided earthy 

 substances proceeds in nature, and the compounds of potassium are 

 everywhere retained by the friable earth in considerable quantities. 

 This explains the presence of so small an amount of potassium salts in 

 the waters of rivers, lakes, streams, and oceans, where the lime and 

 soda have accumulated." 



This "absorptive power of the soil," according to Professor Hil- 

 gard,^ is more displayed in arid than in humid regions. 



The conclusion of the whole matter appears to be that, whereas the 

 sodium compounds tend to accumulate in the waters of the ocean, the 

 potassium compounds tend to be stored in the solid form or retained 

 upon the land, and that to the causes which bring about this separa- 

 tion, and not to any differences in part processes of denudation, the 

 remarkable scarcity in the ocean of potassium relatively to sodium is 

 to be ascribed.'* 



VII. — UNIFORMITY OF DENUDATION BY SOLUTION. 



Land area and rainfall. — The most prominent considerations involved 

 in the question of how far the present rate of denudation by solution 

 may be accepted as an average of that extending over past times are 



^See Roscoe's and Schorlemmer's Chemistry, II., Part I., p. 57; also Mendeleeff's 

 Chemistry, 1897, I, pp. 546, 547. 



'^Quoted by Merrill. Treatise on Rocks, Rock Weathering, and Soils, pj). 369-370. 



* The Palagonite coating on basit; volcanic glass — apparently derived by a hydration 

 and alteration of the glass and the taking up of a small additional amount of potash 

 and soda (apparently from the sea) — is hardly sufficiently abundant, according to 

 present knowledge, to justify consideration here. (See the report on the deposits, 

 p. 304.) The Phillipsite appears to be a purely alteration product of the basic debris. 

 (See Merrill, loc. cit., p. 375.) 



